This invention relates to the treatment of sulfur containing aqueous streams in general and, more particularly, to an economical and environmentally friendly process for oxidizing low valent sulfur compounds in solutions by using SO2, O2 and at least one metal catalyst. The metal catalyst may already be present in the stream, or can be added externally.
Various processes can produce partially oxidized sulfur compounds, such as sulfide (S2xe2x88x92) and thiosulfate (S2O62xe2x88x92) as a waste or by-product. These are typically present in aqueous solution. Depending upon the operating conditions, these species can be further oxidized to tetrathionate, (S4O62xe2x88x92) and trithionate (S3O62xe2x88x92). Collectively, these partially oxidized sulfur compounds are refered to as thiosalts and may be present in solution in varying concentrations. Eventually, these species will be oxidized to sulfate (SO42xe2x88x92); however, this process can be very slow and will be greatly dependent upon the solution temperature, pH and upon the presence of metals or oxidizing bacteria.
The progressive natural oxidation of S2O32xe2x88x92, initially to S4O62xe2x88x92 and S3O62xe2x88x92 and then to SO42xe2x88x92, leads to the formation of acid, which is of concern if these solutions are stored in an impoundment or disposed of to the environment. Also, a high concentration of partially oxidized thiospecies is associated with a high chemical oxygen demand, which is of concern when discharging to surface or ground waters. A minimum concentration of dissolved oxygen is required to support an aquatic ecosystem. The introduction of high levels of thiosalts into natural receiving waters may result in a reduction in the concentration of dissolved oxygen. This would limit the concentration and diversity of life that could be supported in these waters. For this reason, strict controls are placed upon the discharge of streams with a high associated chemical oxygen demand (xe2x80x9cCODxe2x80x9d).
Treatment can be used to reduce the concentration of thiosalts within solution. This may be required prior to recycling solution within the process or to reduce the acid producing potential and chemical oxygen demand associated with the effluent prior to discharge into surface and ground waters. Treatment would involve the oxidation of the various thiosalts to SO42xe2x88x92 and neutralisation with base.
The extent of oxidation of S2O32xe2x88x92 to S4O62xe2x88x92, S3O62xe2x88x92, and SO42xe2x88x92 will effect the final acid producing potential and COD associated with the solution. It should be noted that if S2O32xe2x88x92 is only partially oxidized to S4O62xe2x88x92 and S3O62xe2x88x92, more acid would be produced, per mol of S2O32xe2x88x92 originally present, as a result of the oxidation of each of these species. This would lead to an effluent with a greater acid producing potential. For this reason, the S2O32xe2x88x92 should be completely oxidized to SO42xe2x88x92 when using acid producing potential as the criteria for final effluent quality. When considering the related COD, each subsequent oxidation will result in a slight reduction in the associated oxygen demand; therefore, this may affect the treatment requirements, when using COD as the basis for final effluent quality.
Current treatment technologies employ strong chemical oxidants such as ozone, peroxides, Carro""s acid or hypochlorite. Bacterial oxidation has also been used. These methods, although effective, are often expensive or have severe temperature and operating limitations. In addition, they may introduce unwanted reaction by-products that may cause secondary concerns.
SO2 and O2 have been successfully employed in the treatment of cyanide containing effluents. In this instance, copper is used as a catalyst. Please see Borberly et al., U.S. Pat. No. 4,537,686. Only partial oxidation of sulfide and thiosalts is achieved in this system. In order to achieve complete oxidation of these species, an additional metal catalyst must be added in conjunction with the copper or an alternative catalyst is required.
The process described herein provides a safe, effective and potentially economical way to remove thiospecies from solution. It will not introduce any secondary reaction products that could prove to be problematic, and it has potentially wide application in a number of industrial areas; for example, in the treatment of effluents from pulp and paper mills, sulfide ores mining, photo processing plants, petroleum refining and coking operations, all of which contain thiospecies.
The present invention utilizes SO2 and O2 in the oxidation of low valent sulfur compounds contained in aqueous solution. In the presence of a metal catalyst, all low valent sulfur compounds including sulfide (S2xe2x88x92), thiosulfate (S2O32xe2x88x92), tetrathionate (S4O62xe2x88x92) and trithionate (S3O62xe2x88x92) can be completely oxidized to sulfate (SO42xe2x88x92). Oxidation of thiosalts using SO2 and O2 in the presence of a catalyst offers a safe and effective alternative to the treatment technologies that are currently available.
Treatment takes place at atmospheric pressure and ambient temperature. The process has been demonstrated to work effectively at temperatures from about freezing to room temperature. Moreover, there is a potential for application at higher temperatures.
One or a combination of the transition metals is required to catalyze the oxidation reaction. These metals may be present within the feed or they can be added separately to the reaction vessel. Copper, nickel, cobalt, iron and manganese are among those that were found to work well.
The pH of the reaction media will determine the forms in which these metal species will be present and the relevant equilibrium reactions that will take place. A specific operating pH is associated with each catalytic system. At the proper pH, the active catalytic species will be formed and can be effectively regenerated. The optimum pH will be dependent upon the transition metal(s) employed and ranges from approximately 2 to 11.
Treatment takes place continuously in a vessel, preferably a stirred tank reactor. Sulfur dioxide and air, which provides the oxygen for the reaction (or pure oxygen if available), are added to the reaction vessel on a continuous basis and, in the presence of the transition metal catalyst, serve to oxidize the low valent sulfur compounds that are present in solution. The SO2 dosage will be dependent upon the feed characteristics, catalyst employed, and upon the required effluent quality. The air addition rate and the amount of agitation should be sufficient to ensure that the proper oxygen transfer rates are maintained.
The present invention involves a chemical treatment process for the oxidation and neutralization of low valent sulfur species contained in waste solutions. It can be used as a primary treatment, or it can be used in conjunction with other chemical or physical treatment stages. The waste solution may contain metals and various other contaminants. It can be pumped directly from a process or storage impoundment, and the solution temperature may vary from about 0 to 100xc2x0 C.
The terms xe2x80x9caboutxe2x80x9d or xe2x80x9capproximatelyxe2x80x9d before a series of values, unless otherwise indicated, will be interpreted as applying to each value in the series.
Treatment takes place on a continuous basis following initial batch treatment. During the treatment process, the solution is intimately contacted with sulfur dioxide, oxygen and base. The required reagents are available in many forms; however, for this process, gaseous or liquid SO2, air, and lime are typically preferred.
There are a number of reactions involved in the oxidation of thiosalts to SO42xe2x88x92. Complete oxidation can not be achieved in the presence of SO2 and O2 alone; the kinetics are too slow. Each step must be catalysed. It was found that one or often two different catalytic species are required to achieve a good effluent quality.
Various transition metals can be effectively employed as catalysts. Copper, manganese, iron, nickel, zinc and cobalt can be used either alone or in combination to catalyse the system of reactions. Each of these metals has a given role or serves a specific function within the system, and, in some instances, metals can be used interchangeably. The operating pH is very important and will vary, from about 2 to 11, depending upon the metal or combination of metals employed. Often, the metals required for catalysis naturally occur within the feed solution. If this is not the case, then the required metals can be added externally as solutions of dissolved salts.
The concentration of catalyst will vary depending upon the metals employed and upon the operating conditions encountered. The SO2 dosage, retention time, and oxygen requirements will be dependent upon the feed composition, operating conditions and required effluent quality. Typically, a minimum of approximately 1.5 g SO2/g S2O32xe2x88x92 was required to achieve a good effluent quality. The retention time will be greatly dependent upon the operating temperature. At lower temperatures, a higher retention time is required to compensate for the slower reaction kinetics.
Experimental testwork was initiated to investigate the oxidation of low valent sulfur species in the presence of SO2 and O2. Testwork was designed to investigate the important reaction variables. In each instance, controls were used to demonstrate the independent effect of each variable upon the results achieved.
Synthetic feed solutions were used in all tests. These were prepared by dissolving sodium thiosulfate (Na2S2O3.5H2O) and various metal salts in distilled water. A continuous stirred tank reactor was employed to contact the feed solution with SO2 and oxygen in the presence of the metal catalyst(s). The operating pH and temperature were maintained at a constant value.
Typically, an initial batch treatment was performed. The solution from the batch test was then used as the starting material for continuous treatment. During continuous treatment, feed solution was pumped continuously into the reaction vessel at a controlled rate. The SO2 was added as a solution of Na2S2O5; it was metered in direct proportion to the feed. Compressed air was used to provide the oxygen for the reaction. It was sparged into the reactor through a vertical inlet tube which terminated close to a turbine disposed in the vessel. Air was added, as required, to maintain the dissolved oxygen in the desired range.
Various metals were used throughout this investigation. Some were present within the feed; however, on occasion, the catalyst was added externally to the reactor as a dissolved metal salt solution. A lime suspension was used to control the operating pH. The base was added in response to the measured pH of the reactor.
Samples of treated effluent were collected throughout each test. Each sample was filtered and analyzed for related species. Representative feed samples were also analyzed.
Analyses were performed to determine the COD and the concentrations of S2O32xe2x88x92, S4O62xe2x88x92, S3O62xe2x88x92, SO42xe2x88x92 and STOTAL. Determination of COD involved sample oxidation followed by titration. S2O32xe2x88x92, S4O62xe2x88x92 and S3O62xe2x88x92 were analyzed using calorimetric methods. SO42xe2x88x92 was analyzed using the high pressure liquid chromatography (xe2x80x9cHPLCxe2x80x9d) method, and total sulfur (STOT) was determined gravimetrically. During sulfide oxidation, in addition to those species listed above, S2xe2x88x92 analyses were performed using precipitation with cadmium acetate followed by titration. In all cases, the concentration of metals in the feed and effluent samples were determined using either atomic absorption or ICP.